When aluminum metal is melted, there is formed on the surface of the molten metal a skim or dross comprising a mixture of the reaction products of aluminum with air. These reaction products include aluminum oxide, aluminum nitride and other non-metallic impurities. Prior to tapping or pouring the molten aluminum, the dross is skimmed or raked off the surface of the molten metal. The skimmed dross inevitably contains a substantial amount of entrained metal; often as much as 50-90% by weight. It has been estimated that as much as 1 to 2% of the total aluminum melted ends up in the dross fraction.
Because the dross contains a high percentage of aluminum metal and because the dross fraction represents a substantial processing loss, efforts are ordinarily made to reprocess the dross to recover as much of its aluminum content as possible. Typical processing approaches used commercially today involve the collection, crushing and sizing of dross followed by smelting using a salt flux. Fluxes used typically comprise a mixture of sodium chloride and potassium chloride with small amounts, ordinarily 1% to 5%, of cryolite. Flux is mixed with the dross and charged into a rotating furnace which is heated to a temperature above the melting point of aluminum and of the flux. The salt flux selectively wets the non-metallic components of the dross and promotes the separation of metallic aluminum from the non-metallic fraction. Molten metal is thereafter tapped from the furnace and the residue is then dumped from the furnace. It has been the practice to dispose of the residue in a landfill.
It has been estimated that primary and secondary aluminum plants in the United States generate over 200,000 tons of highsalt residue annually from the processing of aluminum dross. Disposal of these residues by landfill techniques is coming under sharp criticism because ground water leaches the salts from the residue. Furthermore, use of salt fluxes creates corrosive fumes which must be recovered from the smelting operation.